NYC Parks Green Roof A living laboratory for innovative green roof design A project of NYC Parks’ Five Borough Citywide Operations and Technical Services division Randall’s Island, NY June 2014 Introduction In the spring of 2007, the Five Borough Technical Services Division of the New York City Department of Parks and Recreation installed its first green roof system atop their headquarters, the Five Borough Complex on Randall’s Island. Since then, the Technical Services division, in cooperation with the Parks’ GreenApple Corps, has expanded the green roof program to encompass over 30 unique growing systems covering 30,000 square feet of our roof. As the fifth largest green roof in New York City, the 5-Borough project is not only a key component of the Parks’ commitment to reducing its carbon footprint, but it also serves as a way to educate and inspire park staff and patrons, as well as supply a resource for peers in the field. The 5-Borough green roof distinguishes itself from others, as it is the only known green roof in the country to feature distinct systems side by side. These systems vary by type of growing medium, depth of growing medium, and plant selection. By using a variety of planting systems and monitoring their progress, we are identifying the types of green roof systems that will thrive in the NYC urban environment. Background A green roof is a thin layer of vegetation installed on a traditional flat or pitched roof. Although the specifics of green roof design may vary, all green roofs have the same basic components (Fig. 1 from the bottom up): waterproofing layer(s), a root barrier (to prevent the plantings’ roots from growing through the roof), water retention/water drainage layer, growing medium, and plants. Fig. 1: Typical green roof profile There are two types of green roofs: intensive and extensive. Intensive green roofs are greater than 6" deep and allow for a greater variety of plants including shrubs and even trees to be considered. They are similar to traditional rooftop gardens and often provide social and recreational uses. Intensive roofs can require more maintenance than extensive roofs and may also be more costly to install. By contrast, extensive green roofs are much shallower; their growing media are typically less than 6". They are lighter weight systems that provide primarily ecological and economic benefits. Plants likely to flourish on extensive green roofs are more specific and include sedums, succulents, alpine type plants, and some grasses. Extensive green roofs contain fewer layers than those of intensive roofs and tend to require less maintenance. Green roofs vary greatly in weight, depending on depth, material components and if they are wet or dry. The wet weight of a system, measured in pounds per square foot, is defined as the system’s weight when it is supersaturated with water and cannot retain anymore. For each green roof system described in this report, the wet and dry weights are given. Benefits of green roofs Improving water quality: Green roofs reduce the amount of storm water run-off by 50 to 90 percent and reduce the peak flow rate of run-off. During a 1" rainfall, 100% water retention is achievable. Green roofs filter out 95% of the cadmium, copper, and lead and 30% of the nitrogen and phosphorous in storm water. In addition, storm water retention is enhanced by the storm water tanks which store water until it is needed to water the plants. Mitigating the heat island effect: Green roofs cool the surrounding air and thereby reduce the temperature of the mass of hot air that hovers over cities during the summer. When the temperature outside is 90°F, a conventional black roof’s surface temperature is over 150°F, while a green roof’s temperature will vary from 90-95⁰F. Extending the service life of roofs: Green roofs can double or even triple the usable lifetime of roofs. By and reducing temperature variations on a roof (which can cause thermal expansion and contraction stresses) and preventing harmful ultra-violet rays from reaching roofing materials, green roofs may extend the life of a roof by 40-60 years.. Conserving energy: Green roofs reduce the energy required for heating and cooling. In the winter, green roofs insulate the upper floors of buildings and in the summer they cool buildings through shading and the process of evapotranspiration by plants. Removing carbon dioxide from the atmosphere: The plants on a green roof convert atmospheric carbon dioxide, a greenhouse gas, into organic compound by using energy from sunlight. Reducing air pollutants: Green roofs filter airborne particles that can cause respiratory diseases. Reducing sound transmission and reflection: Green roofs reduce the transmission of noise by 5 to 45 decibels and the reflected sound by up to 30 decibels. Creating wildlife habitats: Green roofs create biodiversity by attracting other types of vegetation, animals and insects including butterflies, bees and birds. Improving carbon dioxide/oxygen exchange: Sixteen square feet of vegetation on a green roof produces enough oxygen per year to satisfy the oxygen requirements of one person. Improving aesthetics: Green roofs can add to the beauty of residential and commercial buildings. Sequestering carbon: Green roofs take out carbon from the carbon cycle. For example, one square foot of green roof sequesters thirty-four grams of carbon. Adjacent Solar Arrays: Green roofs cool air around Solar Units, increasing efficiency. Create Green space: Green roofs create recreational areas and green space where there was once no green area. Create an education venue: All of the related benefits of a green roof should be monitored, creating opportunities to educate the public about the benefits of green roofs. Progress of NYC Parks Green Roof Systems Before (2007) After (2013) Aerial Views Before and After Our first installation atop the 5-Borough complex was in the spring of 2007. The system chosen was based on an extensive green roof design provided by Barrett Roofs, a 12" deep monolithic layered system measuring 20' x 40' (800 square feet). The most prominent feature of this system is its 6” layer of growing medium, GaiaSoil™, a low density mixture of compost and recycled expanded polystyrene foam coated with pectin for water absorption. Above the growing medium, a layer of jute mesh was installed to prevent erosion because GaiaSoil™ is structurally unstable and can easily become windborne. The nine species of plants in this system are all native to the New York City metropolitan area and purchased from the Greenbelt Native Plant Center on Staten Island, a division of NYC Parks. Even at an overall depth of 12", this green roof system has a dry weight of only 14 pounds per square foot and a wet weight of 25 pounds per square foot. The estimated cost of this roof is between $25 and $30 per square foot. The second year was a time of major expansion on the 5-Borough rooftop. We installed four extensive green roof systems and five intensive and/or atypical systems in 2008 alone. Two of the systems utilized 2' x 2' trays made from 0.032 gauge aluminum that were fabricated in Technical Services’ own Citywide Shops. The individual trays in this modular system can be temporarily removed, providing easy access to the roof below in case of needed inspections or repairs. Tray Systems I and II each measure 20' x 40' (800 square feet) and use Rooflite™ growing medium, a mixture of heat-treated clay and organic materials. Tray System I is 4” deep and is planted with 440 sedum plugs and 2,000 square feet of sedum cuttings. It has a dry weight of 14 pounds per square foot and a wet weight of 25 pounds per square foot. Tray System II has a depth of 6”, and is planted with 1,520 sedum plugs. It has a dry weight of 22 pounds per square foot and a wet weight of 34 pounds per square foot. The seven sedum species used in both systems were selected for their robustness, contrasting color and texture, and winter interest. A comparison of these two systems over time should provide an indication of the effect of growing medium depth on the viability of sedum green roofs. The estimated cost to furnish and install Tray System I was calculated to be $16 to $18 per square foot and the cost for Tray System II is $18 to $20 per square foot. Green Paks™ modular system was also installed in 2008. Manufactured by Green Roof Blocks of Florissant, Missouri, Green Paks™ are weaved bags made of high density polyethylene measuring 20" x 32" x 4" deep. They are pre-filled with a proprietary growing medium consisting of 80% expanded shale and 20% organic composted pine bark. Green Apple Corps members cut six evenly spaced holes in the polyethylene bags and inserted sedum plugs in to each hole. The 800 square foot Green Paks™ system lies atop a root barrier and drainage mat, and costs about $22 per square foot to furnish and install. Its dry weight is 12 pounds per square foot and its wet weight is 18 pounds per square foot. The fourth modular green roof system installed in 2008 was BIOtrays™, by Green Roof Solutions. These trays are made of coconut husk fiber and measure 17" x 17" x 2”. They were purchased empty, filled with mineral soil and planted with three varieties of sedum. Eventually, the BIOtrays™ break down to form a monolithic system and their decaying components will nourish the mineral soil by providing additional organic matter. The BIOtrays™ cover an area of 200 square feet and the approximate cost to furnish and install this system is $10 per square foot.